(1) Field of the Invention
The invention generally relates to a television system for transmitting picture signals from a transmitter station to a receiver station through some transmission medium, in which system these picture signals are packed in a channel bit stream.
More particularly, the invention relates to a transmitter station and a receiver station for such a television system in which the original quantity of picture data is reduced in the transmitter station by means of a coding operation and the original information is regained in the receiver station by means of a decoding operation which is inverse to the coding operation.
Said transmission medium may be the atmosphere, but also a video tape or a video compact disc.
The invention also relates to a channel bit stream which represents in a digital form the picture signals to be transmitted, and to a storage medium on which said channel bit stream is stored and to a video recorder device adapted to generate and store said channel bit stream and/or display it.
(2) Description of the Prior Art
As is generally known, a television picture is completely defined by three picture signals PS(1), PS(2), PS(3). These may be the three primary color signals R, G, B or, which is the same, one luminance signal Y and two color difference signals CHR(1) and CHR(2) which are sometimes denoted U and V, sometimes by I and Q, respectively, while many other indications are in use too.
For transmitting a television picture in a digital format, the television picture is considered to be a two-dimensional matrix consisting of M rows with N pixels each and only the values of the three picture signals associated with these M.times.N pixels are transmitted to the receiver. These values of the picture signals will be further referred to as picture signal samples and more particularly the terms luminance samples Y and color difference samples CHR(1) and CHR(2) will hereinafter be used.
In a 625-line TV picture, the visible part of each picture comprises 576 lines with 720 pixels each. If each luminance sample of such a pixel is represented by, for example an eight-bit code word, approximately 3.times.10.sup.6 bits are required for representing all luminance samples only, which means that there is a bit rate of approximately 75.times.10.sup.6 bits/sec in the case of 25 frames per second. This is inadmissibly high in practice. The object envisaged by the researchers is to realise a bit rate of approximately 20 Mbit/sec for recording digitized video signals on a magnetic tape or another storage medium.
To achieve said object, the series of television pictures is subjected to some coding operation. Many widely different coding operations are possible and in use. All of them have the property that they make the information to be transmitted available in clusters of code words, these clusters comprising very important code words which may absolutely not be lost (further referred to as first code words) and less important code words (further referred to as second code words) whose loss is less dramatic. The number of code words per cluster may differ from cluster to cluster and the number of bits may differ from code word to code word. In other words, the clusters vary in length. For good order's sake, it is to be noted that the number of second code words in a cluster may be zero. A cluster of code words will hereinafter be referred to as transmission bit block.
An important class of coding operations is the transform coding (see for example Reference 1 in section C). A picture to be coded is partitioned into a plurality of sub-pictures of E by E pixels each. A usual value for E is eight so that a picture is partitioned into 6480 sub-pictures. Each sub-picture is subjected to a forward two-dimensional transform (for example, a discrete cosine transform) and converted into a coefficient block of 8 by 8 coefficients. The most important coefficient is a measure of the average luminance (or colour difference value) and is therefore referred to as dc coefficient. The other 63 coefficients which describe details of the sub-picture are referred to as ac coefficients and their importance generally decreases as the spatial frequency which they represent increases.
Such a coefficient block is further first subjected to some quantization operation and subsequently to some variable length coding. Each coefficient block is thus converted into a transmission bit block consisting of a series of serial data words.
In transform coding, the picture to be coded may be the television picture itself (this is referred to as intraframe transform coding) but also a difference picture, which is produced by subtracting two successive television pictures from each other or by subtracting a prediction picture from a received television picture which is derived from the transmitted difference pictures by means of a prediction circuit. Motion compensation may then be used (see Reference 7 in section C). This is referred to as interframe transform coding. The coding operation may also comprise a combination of intra and interframe transform coding, with a sub-picture being subjected to interframe transform coding if there is little motion in this sub-picture and to intraframe transform coding if there is much motion.
Another coding operation is known as "Adaptive Dynamic Range Coding" (see Reference 8 in section C). In this operation the smallest picture signal sample for each sub-picture as well as the differences between this smallest picture signal sample and the other picture signal samples after they have been subjected to some variable length coding are transmitted as one transmission bit block. The most important code word in this transmission bit block is the code word representing the smallest picture signal sample.
Without further going into detail, it is to be noted that yet another coding operation is known under the name of "Sub-band Coding".
To regain the original picture at the receiver station, the received transmission bit blocks are subjected to a decoding operation which comprises, inter alia a variable length decoding and with which a number of operations are performed on the received transmission bit block which are inverse to the operations performed at the transmitter station so that the original picture is obtained again.
Although a considerable bit rate reduction is realized by means of variable length coding as compared with fixed length coding, the series of data words thus obtained is however very sensitive to transmission errors. A transmission error generally involves loss of synchronization at the receiver station. This means that the separate data words are not recognized as such. The result is a seriously distorted video picture.